The present invention relates to a thermoplastic resin composition having excellent heat distortion resistance, impact resistance and thermal discoloration, resistance, and more particularly to a composition containing (I) a resin obtained by polymerizing a monomer mixture having a high content of .alpha.-methylstyrene in the presence of a diene rubber in a specific manner and (II) an aromatic polycarbonate.
An acrylonitrile/butadine/styrene copolymer (ABS resin) is an inexpensive material having excellent mechanical property and processability, but the use thereof for members requiring heat resistance and higher impact resistance is restricted. On the other hand, an aromatic polycarbonate has been employed as a material which is tough because of having a high impact strength and moreover is excellent in heat distortion resistance. However, the polycarbonate has the defects that thick moldings made thereof are brittle due to poor processability and also it is expensive. For offsetting these defects, it is proposed to blend a polycarbonate with other resins, e.g. a blend of a polycarbonate and ABS resin (Japanese Examined Patent Publication No. 15225/1963), a blend of a polycarbonate and a styrene/methacrylate or styrene/methyl methacylate/acrylonitrile copolymer grafted on a styrene/butadine copolymer (Japanese Examined Patent Publication No. 71/1964) and a blend of a polycarbonate and a thermoplastic vinyl resin containing a linear vinyl polymer made of .alpha.-methylstyrene as a main component (Japanese Unexamined Patent Publication No. 504/1977). However, these known compositions are ones proposed with the objects of improving only the processability of polycarbonate, and accordingly they cannot exhibit both of an excellent heat distortion resistance and a high impact strength over a wide range of the blending ratio. That is to say, in case that a composition having an impact resistance and a heat distortion resistance both maintained on a high level is required, the proportion of the polycarbonate must be raised, and accordingly, in such a case, these compositions have the disadvantages of increase in cost and lowering in moldability. Also, when the content of .alpha.-methylstyrene in a thermoplastic resin to be blended with polycarbonate is increased for the purpose of raising the heat resistance in a low blending ratio region of polycarbonate, the impact resistance of the blend is lowered and also the polymerization conversion in the preparation of the thermoplastic resin is lowered. On the other hand, when a rubber component in the thermoplastic resin is increased for raising the impact resistance, the heat resistance of the blend is lowered.
It is an object of the present invention to provide a thermoplastic resin composition having excellent heat distortion resistance, impact resistance and processability and being inexpensive.
The above and other objects of the present invention will become apparent from the description hereinafter.